Constitutive Analysis and Processing Map for Hot Working of a Ni-Cu Alloy

Gholam Reza Ebrahimi,Amir Momeni,Seyed Mehdi Abbasi,Hossein Monajatizadeh 대한금속·재료학회 2013 METALS AND MATERIALS International Vol.19 No.1

The hot deformation behavior of a Ni-Cu alloy was studied using hot compression testing in the temperature range of 950 °C-1150 °C and at strain rates of 0.001 s.1-1 s.1. Flow curves at low strain rates, up to 0.01 s.1, were typical of DRX characterized by a single peak, while at higher strain rates, the typical form of a DRX flow curve was not observed. The power-law constitutive equation was used to correlate flow stress to strain rate and temperature, and the apparent activation energy of hot deformation was determined to be about 462.4 kJ/mol. The peak strain and stress were related to the Zener-Hollomon parameter and the modeling formula was proposed. The dependence of flow stress to the Z changed at ln Z=38.5,which was considered to be a critical condition for the change in the mechanism of dynamic recrystallization. The efficiency of power dissipation was determined to be between 10-35 percent at different deformation conditions. According to the dynamic material model, stable flow was predicted for the studied temperature and strain rate ranges. Highly serrated grain boundaries at low strain rates were considered to be a reason for the occurrence of continuous dynamic recrystallization. On the contrary, at high strain rates, equiaxed grain structure was attributed to the typical discontinuous dynamic recrystallization.

Vibration analysis thermally affected viscoelastic nanosensors subjected to linear varying loads

Ebrahimi, Farzad,Babaei, Ramin,Shaghaghi, Gholam Reza Techno-Press 2018 Advances in nano research Vol.6 No.4

Unwanted vibration is an issue in many industrial systems, especially in nano-devices. There are many ways to compensate these unwanted vibrations based on the results of the past researches. Elastic medium and smart material etc. are effective methods to restrain unnecessary vibration. In this manuscript, dynamic analysis of viscoelastic nanosensor which is made of functionally graded (FGM) nanobeams is investigated. It is assumed that, the shaft is flexible. The system is modeled based on Timoshenko beam theory and also environmental condition, external linear varying loads and thermal loading effect are considered. The equations of motion are extracted by using energy method and Hamilton principle to describe the translational and shear deformation's behavior of the system. Governing equations of motion are extracted by supplementing Eringen's nonlocal theory. Finally vibration behavior of system especially the frequency of system is developed by implementation Semi-analytical differential transformed method (DTM). The results are validated in the researches that have been done in the past and shows good agreement with them.

Nonlocal buckling characteristics of heterogeneous plates subjected to various loadings

Ebrahimi, Farzad,Babaei, Ramin,Shaghaghi, Gholam Reza Techno-Press 2018 Advances in aircraft and spacecraft science Vol.5 No.5

In this manuscript, buckling response of the functionally graded material (FGM) nanoplate is investigated. Two opposite edges of nanoplate is under linear and nonlinear varying normal stresses. The small-scale effect is considered by Eringen's nonlocal theory. Governing equation are derived by nonlocal theory and Hamilton's principle. Navier's method is used to solve governing equation in simply boundary conditions. The obtained results exactly match the available results in the literature. The results of this research show the important role of nonlocal effect in buckling and stability behavior of nanoplates. In order to study the FG-index effect and different loading condition effects on buckling of rectangular nanoplate, Navier's method is applied and results are presented in various figures and tables.

Thermal effects on nonlocal vibrational characteristics of nanobeams with non-ideal boundary conditions

Farzad Ebrahimi,Gholam Reza Shaghaghi 국제구조공학회 2016 Smart Structures and Systems, An International Jou Vol.18 No.6

In this manuscript, the small scale and thermal effects on vibration behavior of preloaded nanobeams with non-ideal boundary conditions are investigated. The boundary conditions are assumed to allow small deflections and moments and the concept of non-ideal boundary conditions is applied to the nonlocal beam problem. Governing equations are derived through Hamilton\' s principle and then are solved applying Lindstedt-Poincare technique to derive fundamental natural frequencies. The good agreement between the results of this research and those available in literature validated the presented approach. The influence of various parameters including nonlocal parameter, thermal effect, perturbation parameter, aspect ratio and pre-stress load on free vibration behavior of the nanobeams are discussed in details.

An investigation into the influence of thermal loading and surface effects on mechanical characteristics of nanotubes

Farzad Ebrahimi,Gholam Reza Shaghaghi,Mahya Boreiry 국제구조공학회 2016 Structural Engineering and Mechanics, An Int'l Jou Vol.57 No.1

In this paper the differential transformation method (DTM) is utilized for vibration and buckling analysis of nanotubes in thermal environment while considering the coupled surface and nonlocal effects. The Eringen’s nonlocal elasticity theory takes into account the effect of small size while the Gurtin- Murdoch model is used to incorporate the surface effects (SE). The derived governing differential equations are solved by DTM which demonstrated to have high precision and computational efficiency in the vibration analysis of nanobeams. The detailed mathematical derivations are presented and numerical investigations are performed while the emphasis is placed on investigating the effect of thermal loading, small scale and surface effects, mode number, thickness ratio and boundary conditions on the normalized natural frequencies and critical buckling loads of the nanobeams in detail. The results show that the surface effects lead to an increase in natural frequency and critical buckling load of nanotubes. It is explicitly shown that the vibration and buckling of a nanotube is significantly influenced by these effects and the influence of thermal loadings and nonlocal effects are minimal.

Application of the differential transformation method for nonlocal vibration analysis of functionally graded nanobeams

Farzad Ebrahimi,Majid Ghadiri,Erfan Salari,Seied Amir Hosein Hoseini,Gholam Reza Shaghaghi 대한기계학회 2015 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.29 No.3

In this study, the applicability of differential transformation method (DTM) in investigations on vibrational characteristics of functionallygraded (FG) size-dependent nanobeams is examined. The material properties of FG nanobeam vary over the thickness based on thepower law. The nonlocal Eringen theory, which takes into account the effect of small size, enables the present model to be effective in theanalysis and design of nanosensors and nanoactuators. Governing equations are derived through Hamilton’s principle. The obtained resultsexactly match the results of the presented Navier-based analytical solution as well as those available in literature. The DTM is alsodemonstrated to have high precision and computational efficiency in the vibration analysis of FG nanobeams. The detailed mathematicalderivations are presented and numerical investigations performed with emphasis placed on investigating the effects of several parameters,such as small scale effects, volume fraction index, mode number, and thickness ratio on the normalized natural frequencies of the FGnanobeams. The study also shows explicitly that vibrations of FG nanobeams are significantly influenced by these effects. Numericalresults are presented to serve as benchmarks for future analyses of FG nanobeams.

Strain-Dependent Constitutive Modelling of AZ80 Magnesium Alloy Containing 0.5 wt% Rare Earth Elements and Evaluation of Its Validation Using Finite Element Method

Mahzad Zohoori Tali,Mohammad Mazinani,Mahmoud Reza Ghandehari Ferdowsi,Gholam Reza Ebrahimi,Mohammad Marvi-Mashhadi 대한금속·재료학회 2014 METALS AND MATERIALS International Vol.20 No.6

In order to evaluate the hot flow behaviour of AZ80 magnesium alloy containing 0.5 wt% rare earth elements,isothermal hot compression tests were conducted at the temperatures range of 250-450 °C under the strainrates of 0.001-1 s1. The flow curves exhibited a peak stress at a small applied strain after which the flowstresses decrease gradually to a steady state flow stress at higher applied strain values representing theoccurrence of dynamic recrystallization. Moreover, the material constants in the Zener-Hollomon constitutiveequation were found to be strain dependent, and the relationships between these parameters and theapplied true strain were well described by fourth-degree polynomial functions. Accordingly, a strain-dependentmodel was developed in order to predict the hot flow behaviour of AZ80+0.5RE magnesium alloy. The truestresses predicted by this model were found to be in good agreements with the experimental results. Furthermore,the constitutive model developed using the experimental results was incorporated in the finiteelement model with which the hot compressive flow curve of the investigated material was modeled. Theresults showed that the accuracy of flow curves calculated by the numerical simulations depends on theaccuracy of the developed model. In order to evaluate the hot flow behaviour of AZ80 magnesium alloy containing 0.5 wt% rare earth elements,isothermal hot compression tests were conducted at the temperatures range of 250-450 °C under the strainrates of 0.001-1 s1. The flow curves exhibited a peak stress at a small applied strain after which the flowstresses decrease gradually to a steady state flow stress at higher applied strain values representing theoccurrence of dynamic recrystallization. Moreover, the material constants in the Zener-Hollomon constitutiveequation were found to be strain dependent, and the relationships between these parameters and theapplied true strain were well described by fourth-degree polynomial functions. Accordingly, a strain-dependentmodel was developed in order to predict the hot flow behaviour of AZ80+0.5RE magnesium alloy. The truestresses predicted by this model were found to be in good agreements with the experimental results. Furthermore,the constitutive model developed using the experimental results was incorporated in the finiteelement model with which the hot compressive flow curve of the investigated material was modeled. Theresults showed that the accuracy of flow curves calculated by the numerical simulations depends on theaccuracy of the developed model.